1. Field of the Invention
The present invention relates to a method used for an induction type power supply system, and more particularly, to a method of adjusting output power of an induction type power supply system.
2. Description of the Prior Art
An induction type power supply system includes a power supplying terminal and a power receiving terminal. The power supplying terminal applies a driver circuit to drive a supplying-end coil to generate resonance, in order to send electromagnetic waves. A coil of the power receiving terminal may receive the electromagnetic waves and perform power conversion to generate DC power to be supplied for a device in the power receiving terminal. In general, the power supplying terminal may operate with full-bridge driving or half-bridge driving. The full-bridge driving means the driver unit at the front-end of the coil outputs two driving signals to the two terminals of the coil, respectively. The half-bridge driving means the driver unit only outputs one driving signal to a terminal of the coil, and the other terminal of the coil is connected to ground or receives a constant voltage.
In general, in the full-bridge driving, two driving signals respectively outputted to the two terminals of the supplying-end coil are inverse rectangular waves. In such a condition, in order to perform power control in the power supplying terminal of the induction type power supply system, the operating frequency of the driving signals may be adjusted to vary the operating point. Please refer to FIG. 1, which is a schematic diagram of the resonant curve of the coil of the induction type power supply system. As shown in FIG. 1, the resonant curve illustrates the relations between the sine-wave amplitude and frequency of the coil signal during operations of the coil, wherein the resonant curve includes a maximum amplitude Amax corresponding to an operating frequency F0, which indicates maximum output power of the sine wave. In order to prevent the system from being burnt out due to excessive output power, the operating frequency is usually controlled to be greater than F0, such as F1-F4 shown in FIG. 1. The frequencies F1-F4 correspond to amplitudes A1-A4 of the sine wave on the resonant curve, respectively.
As can be seen from above, the coil may output higher power when operating in a lower operating frequency; and output lower power when operating in a higher operating frequency. Therefore, when the induction type power supply system has no load or a light load, the coil may be controlled to operate at a higher operating frequency (e.g., F4) to drive the load with lower output power (the lower amplitude A4 of the sine wave), in order to prevent redundant power consumption. When the load of the power receiving terminal increases such that the power requirement also increases, the operating frequency may gradually be reduced to F3, F2 or F1, in order to enhance the amplitude of sine wave and the output power to driver the load. The process of adjusting the operating frequency is performed via communications between the power receiving terminal and the power supplying terminal. For example, when the power receiving terminal detects an increase of the load, the power receiving terminal may transmit related information to the power supplying terminal by using signal modulation technology, and the power supplying terminal may increase the output power when receiving the information. After the power adjustment of the power supplying terminal is completed, the power receiving terminal may determine whether the updated power is sufficient to drive the present load. If the power is still insufficient, the power receiving terminal then transmits further information to the power supplying terminal, to notify the power supplying terminal to further increase the output power. In other words, when the load varies, power adjustment may not reach its target at one go; instead, the most appropriate power level may be achieved after several times of information exchange between the power supplying terminal and the power receiving terminal. Therefore, the above method is always time consuming and has a defect of poor stability in the output voltage.
Please refer to FIG. 2, which is a waveform diagram of signals when the load of the power receiving terminal increases. FIG. 2 illustrates waveforms of an output voltage Vout of the power receiving terminal and a coil signal Vc of the supplying-end coil. First of all, the power receiving terminal has a light load or no load, where the output voltage Vout remains at a predetermined voltage and the oscillation amplitude of the coil signal Vc is smaller. At time t0, a sudden load appears and forces the output voltage Vout to fall instantly. Due to the resonant effect of the load, the amplitude of the coil signal Vc may increase instantly. When the power receiving terminal detects the load variation (e.g., by detecting the output voltage Vout), the power supplying terminal does not know this information yet and may not increase the output power immediately. At this moment, the power receiving terminal may modulate and/or encode the data indicating the output voltage Vout and then transmit the data to the power supplying terminal (i.e., time t1). When the power supplying terminal receives the modulation data from the power receiving terminal, the power supplying terminal may adjust operating frequency of the coil to increase the output power, in order to adapt to load variations. However, the increase of the output power still cannot make the output voltage Vout return to the predetermined voltage value; hence, the power receiving terminal transmits another indication to the power supplying terminal, where the indication has data or information indicating a further increase of the output power (i.e., time t2 and t3). The power supplying terminal may gradually increase the output power, until the output voltage Vout reaches the predetermined voltage value. In general, since the modulation data is transmitted periodically, the adjustment of the output power should go through several periods of transmissions of the modulation signal, to allow the output voltage Vout to return to the predetermined voltage value.
If the induction type power supply system has to drive a larger load, a rectangular driving signal having larger amplitude is necessary, to generate larger amplitude on the sine wave signal of the coil. The larger amplitude on the driving signal raises the resonant curve of the coil upwards. Please refer to FIG. 3, which is a schematic diagram of resonant curves of the coil under driving signals having different voltage amplitudes in an induction type power supply system. FIG. 3 illustrates the cases where the driving signals have voltages equal to 5V and 24V. Comparing between these two cases, larger output power is achieved when the driving signal has a voltage equal to 24V. In such a condition, when the induction type power supply system is on standby (i.e., there is no load), larger driving signals may always generate more virtual power, which results in more waste power unless the coil operates in a higher operating frequency. However, due to the operating limitation of the driving elements, the frequency of the driving signals must have an upper limit, and a higher operating frequency is accompanied by more frequent switching of the elements, causing a higher wear and tear on the elements to reduce the life of the elements.
Thus, there is a need to provide another power adjustment method for the induction type power supply system, in order to realize fast power adjustment and also prevent the above drawbacks.